MEXICO CITY – Storms caused a big spike in the number of trees blown down or severely damaged in forests where migrating monarch butterflies spend the winter in central Mexico, experts reported Tuesday.

The March tempests caused the loss of 133 acres (54 hectares) of pine and fir trees in the forests west of Mexico City, more than four times the amount lost to illegal logging. It was the biggest storm-related loss since the winter of 2009-10, when unusually heavy rainstorms and mudslides caused the destruction of 262 acres (106 hectares) of trees.

"Never had we observed such a combination of high winds, rain and freezing temperatures," monarch expert Lincoln Brower said of the storms of March 8-9.

Two big storm losses within five years may suggest changes in the climatic conditions that have allowed the survival of patches of mountaintop forests. An additional 16 acres (6.5 hectares) of trees were lost to drought this year.

"This points up just how fragile these forests are, and how fragile the monarchs are, and it makes clear the importance of reforestation efforts," said Omar Vidal, director of the conservation group World Wildlife Fund Mexico.

The monarchs depend on finding relatively well-preserved forests, where millions of orange-and-black butterflies hang in clumps from the boughs. The trees, and the clumping, help protect the butterflies from cold rains and steep drops in temperature.

That is why illegal logging in the 33,484-acre (13,551-hectare) nucleus of the reserve is so damaging. Conservationists have tried to convince the largely impoverished farm and mountain communities that actually own most of the land that the forest is worth more to them in terms of tourism when left standing than when it is cut down for logs.

In April, Mexico's government announced it would create a special national police squad to patrol nature reserves and fight environmental crimes. While the force has not yet formally deployed, illegal logging in the monarch reserve dropped this year, from almost 49.4 acres (20 hectares) in 2015 to about 29.6 acres (12 hectares).

Unlike in past years, when most logging was done in the farming communities, about three-quarters of the tree-cutting this year occurred on public lands in the reserve's core area — precisely the kind of terrain that environmental police could most effectively protect.

"This is why we insist that illegal logging in the reserve has to be eliminated, and that the destruction of (the butterfly's) milkweed habitat in the United States has to be stopped, so that the monarchs have the ability to better respond to these extreme climate events" like the March storms, Vidal said.

He wrote that the "decision to authorize the very extensive salvage logging was possibly the worst management mistake that could have been made."

"The photos I have seen of hundreds of logs on trucks coming out of the reserve, and of huge stacks of piled carefully cut logs below (the butterfly reserve of) Rosario, are atrocious."

The damage comes after a rebound for the monarch. The area covered by the butterflies this winter was more than 3 1/2 times that of a year earlier. They clump so densely in the pine and fir forests that they are counted by the area they cover rather than by individual insects.

The number of monarchs making the 3,400-mile (5,500-kilometer) migration from the United States and Canada had been declining steadily before recovering in 2014. This winter was even better. In December, the butterflies covered 10 acres (about 4 hectares), compared to 2.8 acres (1.13 hectares) in 2014 and a record low of 1.66 acres (0.67 hectares) in 2013. That's still well become the 44 acres (18 hectares) the covered 20 years ago.